New advancements in projection systems utilize an optical semiconductor known as a digital micromirror device. A digital micrometer device chip may be the world's most sophisticated light switch. It contains an array from 750,000 to 1.3 million pivotally mounted microscopic mirrors. Each mirror many measure less than ⅕ of the width of a human hair and corresponds to one pixel in a projected image. The digital micromirror device chip can be combined with a digital video or graphic signal, a light source, and a projector lens so that the micromirrors reflect an all-digital image onto a screen or onto another surface.
Although there are variety of digital micromirror device configurations, typically micromirror are mounted on tiny hinges that enable each mirror to be tilted either toward the light source (on) in a projector system to reflect the light or away from the light source (off) creating a darker pixel on the projection surface. A bitstream of image code entering the semiconductor directs each mirror to switch on or off several times per second. When the mirror is switched on more frequently than off the mirror reflects a light gray pixel. When the mirror is switched off more frequently than on the mirror reflects a darker gray pixel. Some projection systems can deflect pixels enough to generate 1024 shades of gray to convert the video or graphic signal entering the digital micromirror device into a highly detailed grayscale image. In some systems, light generated by a lamp passes through a color wheel as it travels to the surface of the digital micromirror device panel. The color wheel filters to light into red, green and blue. A single chip digital micromirror vice projector systems can create at least 16.7 million colors. When a prism is used to divide a light source into red, green and blue light and three digital micromirror device chips are utilized, more than 35 trillion colors can be produced. The on and off states of each micromirror are coordinated with the three basic building blocks of color: red, green and blue to produce a wide variety of colors.
A variety of digital micromirror devices (DMD) are known. FIG. 1 illustrates one embodiment of a prior art DMD that may be used in the present invention with the substitution of a unique mirror structure according to the present invention. As shown in FIG. 1, a DMD 10 may include a semiconductor device 12 such as a CMOS memory device that includes circuitry 13 that is used to activate an electrode(s) in response to a video or graphic signal. A first layer 14 is formed over the semiconductor device 12 and may include a yoke address electrode 16, and vias 18 formed therein down to the circuitry 13 on the semiconductor device 12, and a bias-reset bus 20. A second layer 22 is formed over the first layer 14 and may include a yoke 24 torsion hinge 26 and mirror address electrodes 28. A micromirror 32 is formed over the second layer 22 and positioned so that the micromirror 32 may be deflected diagonally when one of the electrodes 28 is activate by the semiconductor device 12. The micromirror 32 includes a reflective layer typically including aluminum. The DMD 10 shown in FIG. 1 while being an excellent engineering accomplishment is very complex, costly to manufacture and has low manufacturing yield.
FIG. 2 illustrates a prior art projector system 300 that includes an array of micromirrors 302, typically formed on a semiconductor chip. The array of micromirror 302 may be attached to a printed circuit board 304 or similar substrate that include additional microelectronic devices 306, 308 to perform video processing of video or graphic signal and scaling of the image to be projected. A bright light source 310 is provided and a first optical lens 312 may be present and positioned to direct light from the source 310 through a color wheel 314. The color wheel 314 includes transparent sections with different color filters such as red, green and blue filters. Additional color filters and clear sections may be provided on the color wheel 314. Light emitted from (or passing through) the color wheel 314 may be focused by a second optic lens 316 onto the array of micromirrors 302 so that each micromirror is operated to selectively reflect (or not) the light projected thereon. Light reflected from the array of micromirrors 302 may be focus by a third optic lens 318 onto a wall or screen 320.
A variety of different micromirror configurations are known to provide pivotal movement of the micromirrors. Huibers et al U.S. Pat. No. 6,396,619 discloses a deflectable spatial light modulator including a mirror plate that is substantially ridge and may be made up of a laminate having layers of silicon nitride and aluminum. In one embodiment, the mirror laminate may include a layer of aluminum sandwiched by two layers of silicon nitride. In other embodiments, include only a layer of aluminum and a layer of silicon nitride is provided. Multi-layer arrangements with multiple layers of aluminum and/or silicon nitride are disclosed. The reference states that other materials besides aluminum (such as conductive and reflective metals) could be used. Other materials besides silicon nitride, such as silicon dioxide are also disclosed. The reference discloses that the silicon nitride layer may be 1400 A thick and that the aluminum layer may be 700 A thick. Disclosed also is one or more dielectric films, that act as a reflective coating, may be deposited on the mirror laminate to improve reflectivity.
The present invention provides alternatives to and improvements over the micromirror, DMD and projection systems of the prior art.